Telegraph concentration board circuits



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Patented Feb. 20, i951 TELEGRAPH CONCENTRATION BOARD CIRCUITS Matthew R. Purvis, Fanwood, N. J., assignor to Bell Telephone Laboratories, Incorporated, New York, N. Y., a corporation of New York Application April 29, 1949, Serial N o. 90,513

8 Claims.

This invention relates to telegraph systems and more particularly to telegraph systems wherein private line facilities are associated together in local, statewide, or nationwide networks on a semi-permanent basis as distinguished from a telegraph exchange switching system.

An object of this invention is the improvement of arrangements for associating telegraph circuit elements such as lines and station loops in telegraph networks on a semi-permanent basis in order to facilitate additions to and subtractions from the network and to facilitate the substitution of new circuit elements for existing circuit elements as well as the testing of the various circuit elements in telegraph networks.

Another object of this invention is the provision of means for monitoring on subscriber loops and for testing loop repeaters in the service board ofce by sending test signals in both directions through these repeaters.

The operation of telegraph systems wherein private telegraph lines are associated together in networks which remain interconnected for `in definite periods has been characterized by a lack of iiexibility prior to the development of the inverse neutral form of hub transmission, that is to say, the addition to the network or the subtraction from the network of telegraph lines or station loops, or the substitution in the network of one telegraph line or station loop for another previously required compensatory adjustments of one or more of the telegraph repeaters in the network because the repeaters were arranged in series and the current in the various repeaters was affected when the resistance was changed as the network was altered. Furthermore, added repeaters had to be adjusted to provide the proper polarity as well as type of operation, such as half or full duplex operation.

To overcome this di'iculty, a system of inverse neutral hub transmission was devised and the operation of this system is described in Patents 2,055,277, F S. Kinkead et al., October 3, 1936, 2,069,224, G. C. Cummings, February 2, 1932 and 2,069,251, F. S. Kinkead, February 2, 1937.

This invention differs from the one described in the above patents in that it makes use of a system of electronic hub transmission which employs an electronic control circuit as a means for interconnecting the legs of each line or loop repeater to the telegraph network. This form of transmission is described in the patent application of J. R. Davey, Serial No. 48,491, filed September 9, 1948, now Patent No. 2,528,120 dated October 31, 1950.

fIn the arrangement of the present system, the send and receive leg of each line or loop repeater is extended to a concentration jack circuit which includes two jacks, appearing in the telegraph service board, and a relay. The upper jack which is designated Legs permits the line or loop facility to be split off from its telegraph network or concentration group and patched elsewhere as a substitute or for testing purposes. The lower jack designated Test provides for connection of a test cord circuit such as a telegraph or teletypewriter cord circuit for monitoring by or communication with the service board operator. This jack also permits a spare line or loop facility to be substituted for the assigned one in which case the substitute facility uses the control circuit which was associated with the assigned facility. The test jack also provides access to the facility for the observance of hits `and other disturbances.

From the concentraton jack circuit the send and receive legs of the repeater extend to the leg side of a control circuit. The opposite side of the control circuit connects to the hub circuit in which there are two hubs. Each concentration group includes a receiving hub and a sending hub. Each control circuit connects to the receiving hub circuit via a receiving hub lead designated RH. Likewise each control circuit connects to the send hub circuit via a sending hub lead designated SH. If there is no regenerator in the concentration group the receiving hub is connected directly to the sending hub by means of a hub link so that in effect the two hubs become a single hub. If a regenerator is provided the receiving and sending hubs are interconnected by means of a one-way regenerative repeater which receives signals from the receiving hub, regenerates them and transmits them to the sending hub. Signals transmitted by the receiving relay in a line or loop repeater enter the control circuit via the RL lead and in this case transmission is on a low impedance basis and of the type normally employed in conventional telegraph circuits. Signals received from the line or loop repeater are converted in the control circuit to a comparatively high impedance form of transmission and repeated to the receiving hub via the RH lead. Such signals pass either directly from the receiving to the sending hub via a hub link circuit or through a regenerative repeater. From the sending hub the signals pass over the SH leads to the hub sides of all control circuits to which the sending hub is connected. Each control circuit, with the exception of that which is transmitting to the receiving 3 hub, converts the signals received from the sending hub from the high impedance form of transmission to the low impedance form and repeats them via the SL lead to the send relay in the associated line or loop facility.

Each loop facility which is to be connected into a concentration group is terminated in a loop repeater in the telegraph office where the tele.- graph service board is located. The leg sides of line and loop facilities are identical and every facility requires the use of a control circuit. The sending and receiving hub leads of .each concentration group terminate on the tip and ring'conductors respectively of a two-jack circuit which has been designated hub jack circuit. The lower jack of this circuit is called the Hub jack and this provides for the connection of telegraph or teletypewriter cord circuits for monitoring and .communication purposes. The lower or Hub jack also permits the addition of line or loop facilities to the concentration group. The upper or hub regenerative repeater Hub RGN jack provides for the substitution of a regenerative repeater when one is assigned to the group or for the addition of a regenerative repeater when regeneration is not normally provided.

One of the principal functions of the control circuit is to prevent signals transmitted into the receiving hub by a repeater from being transfmitted from the sending hub back to the send leg of the same repeater. Thus, Whenever a spacing pulse is transmitted through a control circuit to a receiving hub the control circuit functions to prevent this spacing pulse from being transmitted from the sending hub back to the repeater from which it was received. in this case the control circuit sends a mark to its associated repeater from which it is receiving a space. There is one exception to this rule and that occurs when two or more repeaters in the same concentration group transmit spaces toward the receiving hub simultaneously. Spacing signals are transmitted to all repeaters associated with the concentration group during the simultaneous occurrence of two or more spaces. This condition is explained in conjunction with the so-called double-space b31- pass feature.

The purpose of the double-space by-pass feature is to cause the prompt garbling or mutilation of the local copy of any sending teletypewriter station in the event that another station in the same concentration group begins to send simultaneously.

The invention may be more fullyV understood from the following description when read with reference to the associated drawings in which:

Fig. l represents a concentration group comprising three legs or facilities. The concentration jack circuits of only two of the legs are shown and only one control circuit is shown in detail;

Fig. 1A shows a spare control circuit which would be required for adding an additional leg' to the concentration group;

Fig. 1B shows a patching cord circuit;

Fig. 1C shows a release plug circuit;

Fig. 1D shows a by-pass regenerator circuit;

Fig. 1E shows a plug type hit indicator circuit;

Fig. 2 represents a test signal cord circuit which is used for connecting a source of teletypewriter test signals to the Legs jack of a 'writer jack provides for the connection of the 4 attendants teletypewriter cord circuit for communication with the distant attendant or for observing transmission of outgoing signals;

Fig. 3 represents a test signal supply circuit` with associated multiple sender or other source of test signals;

Fig. 4 represents an attendants teletypewriter cord circuit for monitoring or communication in any Legs, Test or l-Iub jack. Jack TMS is provided for connecting a transmission measuring set to the teletypewriter cord circuit;

Fig. 5 represen-ts an attendants manual teiegraph Vcord circuit for monitoring or communication in any Legs, Test or Hub jack. This circuit is also equipped with a TMS jack for the connection of a transmission measuring set;

Fig. 5A shows a calling in signal circuit;

Fig. 6 represents a loop monitoring circuit which provides means for monitoring onths loop side of a loop repeater and for sending test signals toward the hub from the loop` side of the loop repeater;

Fig. 6A shows a loop and equipment jack cir-V cuit; and

Fig. 6B shows a loop patching cord circuit.

Concentration group ated, each line repeater is assumed to .be capable Y of operating full duplex to another telegraph office in which case a line facility interconnects the two offices. In the -case of a facility which extends from the telegraph office to a subscriber station, a loop interconnects the station and the telegraph oiiice and a loop repeater (usually half duplex) is associated with each loop at the telegraph oiice. As the leg sides of line and loop repeaters are identical, the termination in the concentration group is the same for either type of repeater.

The receiving and sending leg of each repeater extends -through resistances to the concentration jack circuit consisting of jacks Legs and Test and relay T. From the concentration jack circuit the legs extend to the leg side of a control circuit. each control circuit being identical with the other, the control circuit of facility A in Fig. 1 being the only one shown in detail. The hub side of each control circuit is connected to the two common hub conductors, the receiving hub lead of each control circuit connecting to the receiving hub and the sending hub lead of each control circuit connecting to the sendinghub.

Although three facilities are shown associated with the concentration group of Fig. 1 the number of such legs or branches is not limited to three but may range from a minimum of two to an unlimited number depending on the impedance of the hub circuit. The receiving hub is held to a potential of +40 volts for the marking condin tion by the potentiometer consisting of battery |34 and resistances H33 and HM. The values Vof resistances ISS and 104 are relatively high so that if a negative or spacing potential. Which may he for'insta'nce 130 volts, is applied to the receiving hub through the same Value of impedance, the hub voltage will become, for instance, -40 Volts. If a regenerative repeater is not furnished, the sending and receiving hubs are strapped to gether through the hub link itt shown at the lower right and the voltage of the receiving hub is impressed on the sending hub. If a regenerative repeater is provided, the hub link It is omitted and the two conductors |61 and |58 are employed. The regenerative repeater responds to signals, voltage fluctuations, on the receiving hub and repeats regenerated signals to the sending hub.

Signals received from facility A actuate polar telegraph receiving relay R of the repeater which connects positive battery to the receiving leg for the marking and negative battery for the spacing condition. It should be noted that the same numbers have been assigned to corresponding items of apparatus in the repeaters and concentration jack circuits of facilities A and B, Fig. 1. The signals pass through resistance 35, in facility A, through the tip and normal spring contacts IEA of the Legs jack, back contacts |62 of relay T and via lead RL to the control circuit. The control circuit of facility A repeats the signals to and receiving hub from which they pass to the sending hub either directly via the hub link or through a regenerative repeater.

From the sending hub the signals are transmitted over the sending hub lead SH of each control circuit associated with the network. The control circuits of facilities B and C repeat the signals to their associated repeaters over the sending legs SL, through back contacts I6@ of relay T, ring spring contacts E65 of the Legs jack, resistance |35 and the line winding of the respective telegraph sending relay S in each facility to positive battery. This causes each S relay to respond to the signals and to repeat them outward over the line or loop facility. The signals received from the A facility are therefore transmitted to all other facilities which are connected to the same network or concentration group. t is important however, that the signals received from facility A are not repeated back to the sending relay of the repeater of facility A as this would interfere with transmission from the sending station. 'I'his undesirable reflection of sign-als is prevented by the control circuit. Operation of the control circuit will now be clescribed.

Operatz'on of control circuit Transmission of signals from the repeater through the control circuit to the receiving hub will first be described. The receiving leg RL of the repeater of facility A which has been traced into the control circuit, continues therein through resistance |23 to the cathode of tube VIA. The branch of RL which extends through resstance lil in the control circuit will be discussed later. Tube VIA is used as a rectifier. Its function is to isolate, during the marking intervai, the receiving ieg RL, of the facility A, which is at +130 volts, from the receiving hub, REC Hub which is held at +40 volts by the hub potentiometer consisting f battery 134 and resistances it and |04. This isolation is effected because the cathode of the tube VIA for the marking condition is mode positive than the plate of the tube. This permits the connection of an almost unlimited number of legs to one hub.

6j since in normal operation only one facility will be effectively connected electrically to the hub. This is While the particular facility is transmitting a --volt spacing signal toward the hub. The one facility which is transmitting therefore toward the hub, and only while it is transmitting a spacing signal, effects a flow of current from the hub through the signal VlA tube in the control circuit associated with the particular facility. During normal transmission all other VIA tubes in other control circuits are blocked for both the marking and spacing signaling conditions and there is therefore no limitation as to the number of facilities connected to a hub. The receiving circuit extends from the plate of tube VIA through the individual control circuit conductor RM to the common receiving nnb REC Hub. The RM leads of all control circuits associated with a given telegraph hub concentration are connected to the receiving hub. Since lead RH of each control circuit is an open circuit for the marking condition due to the condition of the rectifying tube VIA, the combination of a number of RM leads to form a hub wouldhave no voltage on it when all receiving relays are in the marking condition were it not for the before-mentioned hub potentiometer which raises the hub voltage to +40 volts.

When the R relay of the repeater of facility A operates to pacing, 13G-volt battery is connected via lead RL and resistances |35 and |23 to the cathode of tube VIA, causing the tube to conduct. Negative 13u-volt potential is applied through resistance |0| and the plate circuit of tube VEA to the receiving hub. The impedance of this circuit between battery |33 and the hub is approximately 6000 ohms. As the hub potentiometer also has a resistance to ground of approximately 6000 ohms, the resulting hub Voltage for the spacing condition is approximately -40 volts. Thus, the receiving hub voltage +40 for a mark and a -440 for a space. If the receiving relays of two facilities such as facilities A and B, Fig. 1 should go spacing simultaneously, the effect would be the safe as if 13D-volt battery were connected to the receiving hub through a 300C-ohm resistance. In this case,` the hub voltage would be approximately 69 volts. This higher negative hub voltage serves to provide the double space by-pass feature which will be described later.

Transmission from the sending hub through the control circuit to the sending leg of the repeater will now be described.

When a regenerative repeater is not used, the receiving hub is connected directly to the sending hub. The marking and spacing voltages on the sending hub are therefore +49 and 40 respectively.` When a regenerative repeater is used, the output of the repeater is arranged to produce +40 and-40 volts on the sending hub for the marking and spacing conditions, respectively. If a marking condition exists on the sending hub, +40 will be applied via the SH lead and resistance |911 to the grid of tube VIB.

It is on the grid of tube VB that the so-called flip-flop circuit which includes tubes VSA and VSB applies its blocking effect to the sending side of the circuit, either holding the grid positive against a negative spacing signal -lG volts from the sending hub, or allowing the spacing signal to control the tube. It will be assumed at this time, however, that the iiip-op circuit is disconnected and the operation of the sending side 7T' of; the circuitwill be explainedwithout including it. A positive or marking voltage from the sending hub causes tube VIB to conduct. When tube VIB is conducting, its plate voltage is approximately +25 volts and this voltage is applied to the grid of tube V@ .through resistance H8. At this time, three. voltages are applied to the grid oi tube Vd. First, +25 volts from the plate of tube VIB and resistance H8; second, -130 volts from battery |31 andresistance HS; and third, -25 volts through resistance lZ from the plate circuit of tube V2B, which is conducting. Tube VZB conducts at all times except when the re oeiving hub receivesspace vsignals from two or more control circuits simultaneously. The plate voltage rof tube VZB is reduced from +130 for the non-conducting to -25 volts for the conducting condition. The function of tube VEB will be explained in connection with the doublespace by-pass feature. A combination offthe above three voltages applied to the grid of tube Vil through their respective resistances results in a voltage of 22 volts on the grid of tube Vfl, which causes this tube to be hon-conducting. With tube Vil non-conducting, no plate current 'lovvs and accordingly, there is no current in the line winding of the S relay of the repeater of facility A. The l-'mil marking current in the bias winding of the S relay holds this relay in the marking condition. Y

A spacing condition on the sending hub due to a space received from any facility, causes 4 0 volts to be applied through resistance ld to the grid of tube VIB. As the ip-op circuit is as sinned to be disconnected, this +40 volts causes tube VlB to become non-conductive in which condition its plate voltage is +130. This plate voltage is applied to the grid of tube Vd through resistance H8 and it combines with -130 volts applied through resistance I i9 and the -25 volts applied through resistance i2@ to produce a resultant voltage of +22 on the grid of tube V4. This causes tube Vil to conduct.

When tube Vfi is conducting, a plate current,

adjusted to 30 mils by potentiometer i90 which is associated with the screen of the tube, flows in a spacing direction in the line Winding of the S relay and causes this relay to operate to spacing. The +40 volt marking potential on the sending hub 'therefore causes no current to ow in the line winding of the S relay and the -40 volt spacing potential causes a spacing current of 30 mils to new, resulting in inverse neutral trans i sion to the S relay. Tube VIB is employed to invert the positive and negative signals from the sending hub so that for the positive or marking condition of the hub, tube Vfi is non-conducting and for the negative or spacing condition, it is conducting.

The flip-flop circuit will now be described. This circuit is composed of tubes V3A and V3B. The plate of tube V3A is linked to the grid of tube VSB through resistance lll. The plate of tube VSB is linked to the grid of tube V3A through resistance i. The circuit operates on the basis that when tube V3A is conducting, tube V3B is non-conducting and vice versa, and that either condition will be maintained until an external voltage is momentarily applied to flip the circuit, The circuit is comparable to a toggle switch, which has two positions of rest and is changed from' one to the other position by an external force.

To illustrate the action of the flip-flop circuit, it will be assumed that a momentary application of positive voltage to the grid of tube V3AV has caused this tube to conduct in which case its plate voltage is +25. This plate voltage is applied through resistance Ill to the grid oi tube VSB Where it combines with volts applied through resistance IIS to produce a voltage of -50 on the grid of the tube. This causes tube VSB to be non-conducting and the Voltage at its plate is +130. The +130 volt plate potential of tube V313 combines with the other voltages applied to the grid of tube V3A to produce a positive voltage on the grid. With a positive voltage from tube V3B now applied to its grid, tube V3A will continue to conduct after the initial positive voltage which was assumed to have started its conduction, has been removed. Tube VSB, the grid of which is controlled by the condition of tube V3A will accordingly remain non-conducting.

To operate the flip-flop circuit to its other position, a momentary negative voltage will be assumed to be applied to the grid of tube V3A, causing it to become non-conducting. With tube V3A non-conducting, the voltage at its plate becomes +130 and this is applied through resistanceA Ill tothe grid of tube V3B. When combined with the negative potential from battery its and resistance IIG, the Ypositive 130 volt plate. potential of V3A causes the grid of tube V3B to be at 0 potential. As the cathode of'tubevae is at so volt potential the tube will conduct. When tube V35 is conducting its plate voltage is +5. This change in the plate potential of tube V3B from +130 to -5 is applied tothe grid of tube V3A through resistance |09 and this negative voltage swing when combined with the voltages from the ranches which 'include resistances |33 and H2 causes the grid voltage to swing from +27 volts to 8. The resultant negative voltage on the grid of V3A causes this tube to remain non-conducting after the negative voltage which was assumed to make it non-conducting has been removed.

When tube VSB is conducting, the voltage at its plate is -5. This voltage in addition to being connected back to the gridof tube V3A is also conY nected throughresistance |93 to the grid of tube VIB. Since this voltage is small, it `has little erect on the grid of tube VHB and the sending hub voltages i4() volts can control tube VIB as though the voltage from Vthe :dip-flop circuit were not present. With vthe flip-flcp circuit in this condition, with tube' V3B conducting, the passage of signals through tube Vl B is not blocked. When tube V333v is'non-conducting, the voltage at its plate is i130 volts. This high positive voltage, when applied throughv resistance |93 to the grid of tube ViB, is sufcient to keep the grid of the tube positive even -vvhen the voltage applied to the grid from the sending hub is -40 volts. When this condition exists, tube V3B oi the flip-flop circuit holds tube VlB in a continuously conducting condition Y and accordingly blocks the passage of signals on the sending side or" the control circuit.

The summary of the function of tube VSB of the nip-flop circuit in controlling the sending side of the control circuit is that when tube VSB is conducting, the sending side is unblocked and when tube VSB is non-conducting, the `sending side is blocked. l

The'flip-iiop-circuit functions in Such a manner that tube V313 becomes non-conducting When the receiving relay of the associated repeater is transmitting. This applies +130 volts to the grid of tube ViB and blocks the signals from` going over the sending leg to the repeater.

When the receiving relay of a facility other than facility A, such as facility B or C, transmits, the flip-flop circuit of facility A should operate to the condition under which tube VSB is conducting, in which case the voltage applied to the grid of tube VIB is volts and the operation of the tube is not affected. The above functions are accomplished by connecting the grid of tube V3A so that it can be controlled either by the receiving relay of its associated repeater or by a receiving relay of any other repeater connected to the concentration group.

When a space is transmitted to the receiving hub by a repeater other than the one associated with the control circuit of facility A, the shift in hub potential from +40 to +40 volts is applied to the grid of tube V3A through resistance H2. This causes tube V3A to become non-conducting, tube V3B is made to conduct and tube VIB is unblocked. Signals from the other repeater are therefore transmitted from the receiving to the sending hub and through the control circuit and sending leg to the sending relay of the repeater of facility A. When the receiving relay of the repeater of facility A operates to space, it would tend to cause tube V3A to become non-conducting and tube V3B to become conducting due to the negative swing in the potential of the receiving hub. With the flip-flop circuit in this condition there would be nothing to prevent the space from being reiiected back to the sending station.

This condition is corrected and tube VSB is made non-conducting through the action of tube V2A, the purpose 'of which is to convert the positive to negative Voltage swing of the RL leg for a mark to space transition to a negative to positive voltage swing which is applied to the grid of tube V3A to make this tube conduct and tube VSB to be non-conducting.

The action of tube V2A will now be described. When the voltage on the armature of relay R is positive, the voltage on the grid of tube V2A will be positive with respect to the cathode and the tube will conduct. The plate voltage of tube V2A will be +75 at this time. When the voltage on the armature of relay R swings from +130 to +130 volts for a mark to space transition, tube V2A becomes non-conducting and its plate voitage changes from +75 to +130 volts. This correspends to a positive swing of 205 volts. When relay R goes spacing the voltage on the receiving hub changes from +40 to +40 volts and this swing of 80 volts negative is applied through resistance H2 to the grid of tube V3A. The positive 205 volt swing applied through resistance I 08 overrides the negative 80 volt swing applied through resistance H2, however, and the net eifect is that the grid of tube V3A is at a potential of +44 volts and the tube conducts. It should be noted that the flip-flop circuit will remain in this condition with the tube V3A conducting even though the hub potential should fall to +69 volts by reception of a spacing impulse from another facility simultaneously. In this case the potential of the grid of tube V3A will be held to +28 volts and the tube will continue to conduct.

When tube V3A is conducting, tube V3B is non-conducting and the resulting application of +130 volts from the plate of tube V3B to tube ViB holds the latter tube in the conducting condition against +40 volts from the sending hub which is applied to the grid of tube VEB through resistance i951. Signals from the receiving leg of the repeater are thus blocked from going out on 10 the sending leg. When the signals have ceased to be transmitted to the receiving hub from any of the connected facilities, the flip-flop circuit will remain unchanged. The start of signals from another facility will then operate the flipfiop circuit to its opposite condition.

The double-space by-pass feature will now be described.

When relay R of facility A is sending a space to the control circuit, the flip-flop circuit is operated so as to block the passage of signals on the sending side of the circuit. The nip-flop circuit will remain operated in this manner as long as relay R is sending a space even though another sending relay on the hub should also operate to space. This is because the voltage swings on the grid of tube V3A are designed so that relay R of facility A will have control of the flip-flop circuit over the infiuence of the other receiving relays associated with the hub.

It is desirable however, whenever another receiving relay on the hub goes to spacing at the same time as relay R of facility A, that the space signal of the other relay be transmitted from the sending hub to the sending relay of .facility A. This is for the purpose of mutilating the home copy of the distant sender of'facility A. It indicates to the sender that another station has sent simultaneously on the concentration group. This is called the double-space Icy-pass feature and is accomplished by tube VZBwhich provides a by-pass around the block established by the flip-flop circuit.

The action of the by-passicircuit is described as follows:

The cathode of tube V2B is at a potential of +50 volts and the grid is connected to the receiving hub via resistance III. The normal marking and spacing voltages on the receiving hub of +40 and +40 volts leave tube VZB in the conducting condition. In the conducting condition, the voltage at the plate of tube VZB is +25. This voltage is applied through resistance E20 to the grid of tube V4 and since it is small compared to the volts applied through re sistance H9, it has no effect on the operation of tube V4. When two spaces exist simultaneously on the receiving hub, the voltage on the hub becomes +69. This negative voltage when applied to the grid' of tube VZB causes the tube to become non-conducting in which case its plate voltage is +130. The 15S-volt swing in the positive direction +25 to +130 of the voltage applied through resistance |20 to the grid of tube V4 changes the potential on the grid from +22 volts to a positive value and the tube conducts. With tube V4 conducting, the sending relay of the repeater is operated to spacingand a space signal is sent back to the distant point even though that point is sending a spacing signal simultaneously to the control circuit..

The above description of the control circuit of facility A applies to any control circuit assigned to the hub such as that associated with facility B or C. The description also applies to a spare control circuit which might be added to the hub circuit in conjunction with a spare line or loop facility on a patched basis. The voltage values which have been used in the above description are typical voltages and are given merely to explain the manner in which the circuit operates.

Substitution of facilities-Figs. 1, 1A and 1B The concentration jack circuit provides for the substitution of line and loop repeaters, and the hub jack circuit provides for the'substitution of regenerative repeaters. Substitution of these facilities becomes necessary chiefly because of trouble in the facility. Substitution of line and loop facilities is accomplished in the same manner. The method is as follows:

Let it be assumed that a trouble condition exists in the line or loop associated with facility A and that a spare line or loop facility is to be substituted for facility A in Fig, 1. This is accomplished by patching the Legs jack of a suitable spare line or loop facility `to the Test jack of facility A by means of the leg patching cord circuit of Fig. 1B. The Legs jack of the spare facility is similar to that of assigned facility A which is now to be disconnected from the concentration group and replaced by the spare facility. Currentflows from the ground on the sleeve of the Legs jack of the spare facility which isy similar in its jack arrangements to all facilities, such as facility A or B, through-resistance |38, the sleeve circuit of the patching cord, Fig. 1B,sleeve of the Test jack of facility A and through the winding of relay T to battery 139 causing relay T in facility A to operate. Operation of relay T disconnects the RL and SL leads of the facility A, which is to be replaced, from the control circuit. The RL lead from the spare facility now extends from the tip of the spare Legs jack, over the tip conductor of the patching cord to the tip of the Test jack of facility A and over the RL lead to the control circuit. Likewise the SL lead from thespare facility extends from the ring ofthe spare Legs jack, over the. ring conductor of the patching cord, the ring of the Test jack of facility A, make contacts it! of relay T of facility A and the SL lead to the control circuit.. The spare facility is thereby substituted. for the normally assigned facility A. The line or loop repeater ofv facility A is no longer connected to the concentration group but now terminates on the tip and ring of the Legs jack of facility A from which it may be patched for test purposes or for use elsewhere as a spare after the trouble has been cleared.

A spare regenerative repeater may be substituted for either an assigned regenerative repeater or a hub link. The hub jack circuit of Fig. l provides for the substitution of a regenerative repeater to replace an assigned hub link at any time that regeneration is required owing to transmission degradation. lThe method of substituting a spare regenerative repeater for another regenerative repeater is the same as that Vrequired when a hub link is to be replaced by a spare regenerative repeater. The procedure is as follows:

A spare regenerative repeater terminates in a'jack similar .to jack |44 which is designated RGN. The RGN jack of a spare regenerative repeater is patched to. jacky 45 which is ther upper hub jack designated Hub RGN. The patch is made by means of a leg patching cord circuit similar to that of Fig. 1B.. Current ows from battery lill' through resistance |45, the sleeve of jack |44, over the sleeve of the patching cord to Ythe sleeve of jack |45, and through the winding of relay HS to ground. This causes relay vHS `to operate and disconnect the SL and RL conductors of the assigned regenerative repeater or hub link from the sending and receiving hubs.

.At .the same time the SL `lead ,of the spare regenerative repeater will be connected through the tip conductor of the patching cord and the tip of jack M5 to the sending hub; Likewise the RL lead of the spare regenerative repeater will be connected through the ring of the ypatching cord tothe receiving hub. The spare regenerative repeater will therefore replace the assigned regenerative repeater or hub link and it will receive signals from the receiving hub, regenerate them and send them to the sending hub.

Addition of facilities It is particularly pointed out that there is a difference in operation between the substitution of a facility for one already forming part of aconcentration group and the addition of a facility to a concentrationgroup. Substitution was described in the foregoing. In the case of Vsubstitution it should be apparent from the foregoing that the facility which is disconnected is separated from its control circuit and this control circuit is employed by the facility that is substituted for the disconnected facility. When a facility is added, however, as distinguished from substituted, there is no disconnected control circuit, forming part of the original concentration group, available to serve with the added facility and another must therefore be supplied as will become apparent in the following.

Line and loop facilities are added in the same manner and the addition of a facility requires the use of a spare control circuit since it is impractical to assign extra control circuits to a concentration group to anticipate additions, n this description it is assumed that a regenerative repeater is never added, as distinguished from substituted; because if a regenerative repeater is not normally assigned to a concentration group the use of a spare regenerative repeater at some later time on a patched basis amounts to a substitution as the spare regenerative repeater is substituted for the assigned hub link such as |66.

The manner in which line and loop facilities are added will now be described.

The Legs jack of a suitable spare facility, line or loop, such as facility A or B assuming it has been released from service by a substitution as previously described, is patched to the Legs jack of a spare control circuit similar to that shown in Fig. 1A. The Hub jack of the spare control circuit Fig. 1A is next patched either to Hub jack I5@ or the Hub RGN jack |45. Both patches are made using a patching cord similar to that shown in Fig. 1B so that two Figs. 1B Vare required for the addition of a facility. Signals incoming from the added facility will pass through the first patching cord per Fig. 1B, through the RL lead of Fig. 1A into the control circuit, represented by the rectangle in Fig. 1A, and then from the control circuit over the RH lead through the sec- `ond patching cord per Fig. 1B through the Hub jack |56 or Hub RGN jack |45 to the receiving hub. Likewise signals from the sending hub will pass from the sending hub (Send Hub) through jack |50 or |45, Fig. 1B, over the SH lead into the `control circuit in Fig. 1A and, if they are not blocked in the control circuit, they will be transmitted over the SL lead into the second patching cord per Fig. 1B into the legs jack of the added facility. Relay HS of the hub jack circuit will not be operated if the patch has been made to the Hub RGN jack because there is no sleeve connection to the Hub jack of the spare control cir cuit per Fig. lA.

Releasing facilities from the concentration group A line. or loop facility such as that of facility A ment, if desirable, by operation of the half-full duplex key in the cord circuit as will be described later. In the case of a substituted facility the communication cord circuit per Fig. 4 or 5 cannot be connected to the Test jack because a patching cord circuit has been connected to this jack, as explained, in order to effect the substitution. In thisl case the communication cord circuit is connected to the Test jack of the patching cord circuit. Likewise in the case of an addition, the communication cord circuit per Fig. 4 or 5 cannot be connected to the Hub jack because of the patch to the Hub jack necessary to eiiect the addition, as described. In this case the communication cord circuit is connected to the Hub jack of the patching cord circuit for monitoring.

Communication Y ing of the tip and ring contacts of the Legs jack.

The RL and SL leads from the facility are connected to the tip and ring of the communication cord respectively.

2. The Test jack of a concentration jack circuit for communicationv with the concentration group in which case the facility is split oi from i the concentration group by operation of relay T. 3. The Hub jack of a concentration group per Fig. 1 or a patching cord circuit per F'ig. 1B for communication with the group.

The operation of the communication cord circuits will be described later.

Transmission measurements Transmission measurements are made in any concentration group by connecting a communication cord per Fig. 4 or Fig. 5 to a Legs, I-Iub or Test jack of Fig. 1 and patching a transmission measuring set to the transmission measuring jack, TMS jack, of the communication cord circuit shown at the upper right in each of Figs. fi

and 5. This will be described in detail hereinafter in connection with the attendants teletypewriter cord circuit and the attendants manual telegraph cord circuit.

Sending ieletypewriter test signals-Figs. 2 and 3 Teletypewriter test signals appear in the service board in the test signal supply circuit of Fig. 3.

Teletypewriter test signals can be patched to Legs jacks of facilities by either a leg patching cord circuit such as that of Fig. 1B or by means of the test signal cord circuit of Fig. 2. Leg patching cords are always used to connect the test signal supply circuit to Hub jacks.

The testsignal cord circuit provides 'the following features which are of value when test signals are applied to the Legs jack of a facility:

A. Itpermits the attendant at the distant end of the facility who is observing the quality of the signals, to stop the transmission of signals by sending a break signal.

B. When transmission of signals is stopped by reception of a break signal a supervisory lamp is lighted in the test signal cord circuit perFig.

.2 and the auxiliary audible signal sounds, until the local attendant answers by connecting a communication cord circuit per Figs. Llor 5 to the TTY jack shown at the upper right in Fig. 2..,

C. The local attendant may communicate 'with the distant attendant through the test signal'cord circuit per Fig. 2 by means of his communication cord circuit per Fig. 4 or 5 connected tothe test signal cord circuit.

D. The distant attendant may recall they local attendant and cause the supervisory lamp to light and the auxiliary signal circuit to sound in the test signal cord` circuit per Fig. 2 while the local attendants communication cord circuit per Fig. i or Fig. 5 remains connected tothe TTY jack of Fig. 2.

When features A to D above are not required, a leg patching cord circuit per Fig. 1B is used to patch the test signals to a Legs jack. Since these features are usually not required when test signals are applied to the Hub jack of a concentration group, this patch is always made by means of a leg patching cord circuit and'no provision lis made for using the test'signal cord circuit for this purpose.

Test signal cord circuit and test signalsupply circuitl The operation of the test signal'cord circuit per Fig. 2 and the test signal supply circuit per Fig. 3 will now be described in detail. Reiter to Figs. 2 and `3.

The test signal supply circuit is patched to the Legs jack of the facility under test withthe SIG- ,cord of Fig. 2 connected to the SIGS jack of the test signal supply circuit per'Fig. 3 and the Leg cord of Fig. 2 connected tothe Legs jack of the facility per Fig. 1. Signals will not pass through the cord circuit per Fig. 2 to the Legs jack until the start signal key ST SIG has been momentarily depressed. Tube 2&2 Will respond to signals on the tip of the Legs jack in Fig. 1 and if a mark is incoming the receiving relay R of the facility Will connect positive battery to the RL leg and tube 2te will be held conducting. In this state relay P will be held operated as shown in Fig. 2. If a space is incoming over the facility the receiving relay R will connect negative battery to the RL lead and tube 292 will become ytial applied to the tip of cord Leg in Fig. 2 causes condenser 2&1 to begin discharging through resistance 202 and the network consisting of resistances 2&2, 203,225 land'condenser titl is so designed that if the spacing signal persists for approximately oneV second the voltage on condenser ZBI ywill be suihciently negative yas to cause Vtube 291 to become non-conducting by lowering the potential of its grid. This insures that tube "2ii9 will become non-conducting and relay P will be released whenever a two-second spacing signal which is employed as a break signal is received from the facility under test. It also prevents tube 209 from releasing relay P during reception of teletypewriter signals'of .normal durationf-rom the facility. Momentary operation 4of l key ST SIG causes relays ST and SS to operate atrasos over obvious circuits f andito'llock up.' "Relay 'SS Zopera-ted, also vcauses polar relayaand relay' B,-Fig.V 3, to operateso astolclose their resp`eccontacts. Polar relay S ofFig. 3 remains reu leased f'becausefcurrent-"fiow v for this condition is in "the-f Wrong direction *to causes its operation. Relay "ST;'- iFig. f2, locks up with current flow frein ground through itswinding and upper'closed con# tactsjth'e lower operated contacts of relay `P and teryf2`l4.- "Operation'of relaySTcloEea-the transmission"circuitbetweenthetip offcord STG to thelring of cord Leg "which includes resistance 25H: It alsocpens circuits through its fbottom back contactsso to-^prevent thelsuperrisoiy lamp '2i-l frorndightingand ithe auxiliary signal circuit 'from' sounding'at `this time.

Relays A and lFig; in "operating, cause relays Ai 'and AB to operate over obvious circuits. RelaysAirand AB-When operated connect'ground through their bottom make contactsto lead ST Whichzstarts"themultiplelsender 'or `other source of signals Well knowntinthe art, indicated by a rectangle. Test signals transmitted by the multiple sender; n'ow pass 'over'lead T through resistance-222,` upper `inner operated vcontacts `of relay "A lower `inner voperated contacts of 'relay AB'to' the tipof jacklSlGS and through` the' tip of "cord SIG, Fig. 2.1and the upper outer make contacts 'of relay'ST to l'the ring of cord Leg and intothe facility'undertest. The type of `transmissicnisinv'erse neutral. V'The sending -source indicated by the rectangle in Fig: 3 connects positive potential to leadt'IjFig;v 3, for aniark and negative potential'for aspace The line Winding of sendingrelay. Sfinthe' facility per Fig. 1 is terminated in positive"battery"so current does notA i'w through' the'line unnding of this relay for the marking-condition; *The reiay Vis 'operateiiA to markingby itsbiasingwindingg the' lower 'winding in Figpl; Current owstnrougnlthe top Winding ofjrelay S for the spacing condition'and this current Vhas aproxiinately "twicemthe" Value of that ilowing inthe biasing Winding As theline current is in a` spacingdirection, it" operates the sending relay to spacing 'in opposition to the effect ofthe lower. biasing Winding. In passing from the "multiple sender, Mult SDR, Fig.` 3 through tlie'test sgn'alsupply and test signal cord circuits and "over the 'sendingleg SL to the line windingof relays, the spacing 'line current passes through'resistance 222 in the test signal supply circuit, "resistance ^20I 'in thete'st 'signal cord circuitandresistance it in'the SL v'lead ofthe facility'per LFig. 1. These resistances are provided'to adjust thiscurrent to the optimum Value forioperating the S relay. The VS relay will'respond atta standardratetoioperation to the test `signals and repeat them `overthe facility to the Vdistant point 'where the biasl and distortion will befobserved and measured.

VnWhen thedistant attendanthas finished his observationa'he sends a two-second break signal Whicnenteisthe' test signal cord circuit over the tip of cord' Leg'and causes tube"2ll9i to 4stop conducting Tin'fthe "manner previously described.

Relay P releases Aand opens 'the lockingv `circuit of" relay-'ST causing. lits're'lease. `Relay STin *releasi ing Willstop thetransmission ofsignals. Super-` visorylamp 2i-3- Will lightfstea'dily and the auxiliarysignai circuit will soundan alarm. Although tube iiilfiwill conduct-again at the end ofvth'e break, which isla space signal, and thereby causev relay PJ-Ito lreoperate, Vrelay ST will remain in the released; condition;

The local attendant lmay wish to` communicate f with i the --distant. .attendantf before .taking l down th'e-p'atcl'i.l Ifso; lhe connects the teletypewritei o: telegraph -coidwircuit penFigs. 4 or 5 to the lfidjaoklat the/upper rig-ht in Fig. 2. co irnun" aticnli T entire-"normal coilitacts-of-theTTY jack. Relay 1d-zcircuit :is Yconnected tothe When the jaclalrelayngiffigi 2fis operated from ground f A in operating opens the circuit through its bote' tombaclr con-tactthrough which thesupervisory lainp was 'lighted and ther audible signal "Wasi sounded 1as-lieretofore -described. It alsoprepares an alternati-vefcir'cuitfor lighting the super-'f Visory lamp `and sounding .the auxiliary isignal alarmas-scon as relay'P isxagain releasedtlbyian incominglrecall signal which isa spacing signal of-ilongldurationf. IfJsucha-recall-is receivedfthe y lamp andialarm signalswillbe locked in. `'Ihisis acconiplishedlby the fact that -in operating, `relay`- a prepares -a--pathA atlitslup'per make con-tact whereby negativeba-ttery' 2 |12, Fig. 2, is-connected to theigridofltube" Zingas soonas relayP is releasedeby arrincoming? iecallsignal. The contion` after `the sspaceiori recall signal that caused it to becomenon-ccndueting is no longer pre'esen-t.`

This-feature insuresff-that the lampand alarm'Y signals -will respond-tothe -recall signal and continue i to attract attention evenl-though-Athe local attendant has-connectedhis-fteiegraph cord TLG, 5, loi -tel'etypewriter cordgTTYg- Fig. lL-t the test-signalcordicircuit. Theilocalatten'dant may cutofi the Alampandauxiliary signal alarms by l momentarily disconnecting his lcoirununication is,` being received iromthe facility,

"The local attendantmayzrestart the testsignals`-` by momentarilydepressing tliesta'rt signalkey,

"relay ifi,'fl;1ig. 2,K ni'oinentarily fandlpermits tube V20S to become conductive, @providinga marking signa-lf f which*reoperatesrelay ST. With-the signals. "By connecting atransm'issionllmeasuring set; notfshown; -butwell@known` iii-the art, toL the transmission measuring-jack TMS at `1the upper right in theeteletypewriter cord circuit per Fig.` 4, the-bias andfdistorti'on of the outgoingL signals beobserved and measured.

'Upon completion-of -the-tests; the-patch in@ voivling Jtheitest:signal `cord circuit-is taken fclowni Disconnectionof theisignalcord, SIG, Fig. 2, from Y the-signalsfjaclt-JSIGSfFig-S, opens the locking circuit for relay y"ESS, fFig. 2, -and this relayisreleased.

`vihen a 4leg Epatching cord s circuit similar gto that ofFig. lBis usedtoY patch the test signal' supplycircuitper Figf -`to" the Legs jack of'a facility,A current iiotvs from battery 22 l Fig.- 3,y A 'through the windings of relays S, B and NA;

through the Asleeyecircuit` of vthe `leg patching cord circuit per Fig. 1B and through resistance I 3b, *Figs `l, to ground. This current" flow "causes the armature offrelay S, Fig. 3,"`to engageits contachbut'relays Aaricl"B"remainreleasedA as Y 19v the current isn the wrong direction to operate polar relay A and it is not of sufficient value to operate relay B. Operation of relay S connects ground to lead ST in Fig. 3 and starts the source of test signals. The signals immediately pass over lead T, through resistance 222, upper back contacts of relay A|, resistance 2|6 to the ring of jack SIGS, Fig. 3. Continuing over the ring conductor of the leg patching cord circuit per Fig. 1B into Fig. l, the signals pass over the sending leg SL to the S relay of the facility. Transmission is identical in this case to that previously described when the patch is completed using a test signal cord circuit. No provision is made for the distant attendant to stop the test signals automatically when the leg patching cord circuit is used.

A leg patching cord circuit per Fig. 1B may be usedV to connect the SIGS jack of the test signal supply circuit, Fig. 3, to the I-Iub jack of a lconcentration group in Fig. 1, in order to send test signals to the entire group. In this case current flows from battery 22|, Fig. 3, through the windings of relays S, B and A and over the sleeve of the leg patching cord circuit per Fig. 1B, into Fig. lY through the Hub jack and through resistance |43 to battery |42 in the sleeve circuit of Fig. l. Although battery |42 of Fig. 1 is poled so asto oppose battery 22| in Fig. 3, current will flow in the proper direction to operate polar relay A to engage its contact since the voltage of battery 22| is approximately one-half that of battery |42. The armatures of relays B and S will remain disengaged from their respective contacts because the current is in the wrong direction to lactuate the armature of polar relay S to engage its contact and it will be of insufficientvvalue to operate relay B. The closing of the contact of relay A causes relay A| to operate over an obvious circuit. This connects ground through the bottom outer make contact of relay AI to start lead ST, to start the source of test signals; The signals will now pass over lead T, through resistance 222, upper inner make contacts of relay Al, lower back contacts of relay AB, resistance 224, from cathode to plate of tubel2l1, upper outer make contacts of relay A| to the-ring o f jack SIG. Negative battery 220 and resistance 219 are provided to protect the tube 2|1, by preventing the cathode ofthe tube from becoming too positive in potential with respect to thel heater. From the ring of jack SIGS, Fig. 3, the signals pass over the ring of the leg patching cord circuit per Fig. 1B to the receiving hub which terminates on the ring of the Hub jack in Fig. l. Tube 2|1 conducts only during transmission of spacing pulses and is not conducting during marking since for that condition its cathode becmes more positive than its plate which is at the hub marking potential of |-40 volts. The tube therefore operates as a diode and during a mark the source of test signals is isolated from the hub which remains at its normal positive marking voltage. During a space the tube conducts and since its cathode is connected to a negative low impedance source, suicient current iiows from the hub potentiometer, formed by battery |34 and resistances |03 and |04, Fig. 1, to lower the hub voltage to approximately volts.

Attendants teletypewriter cord 'circuit- Fig 4 i The attendants teletypewriter cord circuit is shown in Fig. '4. It is provided-principally for the purpose of connecting the position teletypewriter, having sending contacts 302 and receiving- TMS. In certain applications the sending andv receiving elements of the cord circuit are connected to the sending and receiving hubs 'and transmission is on a high impedance basis. In other cases the sending and receiving elements of the cord circuit are connected to the sending and receiving legs between the control circuit and the repeater in which case communication is on a low impedance basis. matically distinguishes between jacks of the various types on the basis of the magnitude, andv polarity of the sleeve voltage of the jacks. This is accomplished by means of the three relay selecting circu.t including relays A, B and S which are in series in the sleeve of the cord circuit. Op-

erat.on of the teletypewriter cord circuit is .de-

scribed below.

Attcndants teletypewriter cord circuit connected to Hub jack Let us assume that cord 30|, Fig. 4, is .connected to Hubjack |50. of Fig. 1. The sleeve circuitmay be traced from battery |42 through. resistance |43 to thesleeve of hub jack |50 in Fig. '1 and then over the sleeve of the cord 33| through relays S, B and A to battery 321 in Fig. 4. Batteries |42, Fig. l, and 321, Fig. 4,-are poled in opposition and the voltage of battery |42 is approximately twice that of battery 321.

AThe current flow through the windings of relays A, B and S in Fig. 4 is in such a direction as to actuate the armature of polar relay A to engage with its left-hand contact 384.` The contact 386 of polar relay S remains open as the current is not Yin the proper direction to close it.` The current is not suicient to operate marginal relay B, Fig. 4, Relay A when operated connects Vground to the winding of relay AI causing its' operation. Relay AI connects ground through contact 368 to start the teletypewriterI motor.

As relay BI is released at this time, relay A2 will also operate on current which flows from ground through contact 388 of relay AI through contacts 39| of unoperated relay B| andthe wind.

ing of relay A2 to battery 33|. Signals received from the sending hub in Fig. 1 enter the cord cir-^ cuit per Fig. 4 over the tip conductor and pass,- `through resistance 320 to the grid of tube 3|'|.. The grid of tubeY 311 -is brought to a positive pocurrent which is approximately twice the value of the spacing current iiowing in the lower windings" of relays R and RI causes the relays to operate to marking. Relay RI operates the teletypewriter receiving magnet 339 on a neutral basis,

closing and opening a circuit from battery 309 through resistance 308,.its own armature and the winding of magnet 339 for the marking and y The cord circuit auto-vy amazon,

Spacing.: condition,` respectively. BelarR transf mitsfona polar b asis from batteriesl nd, 311 .L transmission,

through resistance 3 I2 to the ltip of measuringsetjack TMS, through theinput. relay ofthe transmissionA measuringv set, :to ground on the-ring of jack TMS. The screen grid of tube 3J? gis connected to the potentiometer;formed"byy battery `3,16 :connected through resistances SI and 3| 4 to ground. This potentiometerisaadjusted .to produce the desired plate .current l through the upper Windingsof relaysR. an'dRl for; the `:marking condition.

' .The position teletypewriter sendssignalsinto the sending hub. `Local copy` is .obtained-fromtheisending circuit which connects` to. .the.tip1.:of

thecordicircuit and `.outgoing signals; are. there fore received by the position l`telletypewriterv through the receiving elementas previously exe plained. In order to Asend .tothe sending rhubjtV isvnecessary 4to operate the? teletypewrter.;send.. key Send TTY. This removes a ground from. the upper windingoi polar sendrelayEwhich i'n .the

idle condition :holds the armature OLthiSirela-y inengagement with its right-hand orcrnarking Contact. It also closesthe sending circuitwhich diode andcompletely isolates the armatureof relay 4El from Vthehub circuit during transmission of a mark. For aspace, the `armature of relay E'connects `negative battery 3% through resist` ances 3D3 and 33S to the cathode of tube332". .In this casethe tube conducts since its cathode is negative with respect to its plate. 'In the--con-'J ducting orspacing conditionfcurrent `flows from the'sending hub through the plate circuit of .tube 32gresistances 33!! and33 to negative battery and'this causes the potential of the nutrito become negative `-due to the voltage'ld-rop acro'ssl the hub potentiometer network. A Anegative voltage on the sending hub constitutes a spacing' condition onthe hub.

1t should be `noted that battery fconnects to {the'cathode of tube 3&2 viaV resistance'333.V

The -pur-posepf this negativebattery and resistance branch is to prevent the cathode from becoming too positive `irl-potential with respect to p the itube heater element when the tube is inthe marking or non-conducting condition.

Teletypewriter cord circuit connected to Leg'jacjc of concentration jack circuit nWhen the teletypewriter cord circuit perFig.

4- isfconnected to the Leg jack of a concentration'` jack'circuit per Fig. 1, current ilows from battery S21-fin Fig. 4 through the windings ofirelays A,

Band S and over the sleeves of the cord `and tlreLegs jack to ground through resistance `i323, Figg'l. This results in actuating thearmature of relays to'engag'e its Contact "386 which connects ground `to start the teletypewriter motor. fThe armature of'relay A remains in engagement with.

itsright-hand contact 382 as shown and relay` B .does'not operate` because the currentiiow `islin i lliV 22:. thegwrorigdirection to foperateV polar relay A. and itis not ofsulicient value to operate mar ginalrelayB. Signals from the facility to which the teletypewriter cordcircuit is connected Will be received on the RL lead ,which terminates on the tip of jack Legs in Fig., l. These signalspass over the tip of the cord to the grid of tube 3 Il via resistance 320. Thering of the cord connectsttov the sending .leg of. the facility: per Fig. 1 which has an positive potential applied through the linewi'nding4 of the S relay in Fig. l. Thering oi' the cord extends through backcontactt'll ofre` lay Ai `in Fig. 4, throughresist'ance 33T, back contact 3&3! `of relayAl, closed contacts 362 of full duplex-halt duplex key FDX--llDXl and resistanceltothe grid of tube 3H. ln viewof the `fact that Aa steady positivepotential is` con-v nected to theA grid of tube 3! l'through the ring' of the cord circuit, thegrid potential lrequiresthe` addition of (compensatory 4negative `potential.`

ingorder` to restore .the Vsymmetry of Vthe receiving element. and permit the equal and opposite potentials `received over the tip conductorfrom the`A facility to produce unbiasedA reception. This isy accomplished by the branch circuit consisting 'of f negative battery 336 which connects to the Vgrid,

throughbackcontacts 3&4 of relay Ai and resist?, ance 3|8. Tube 3i? conducts for a marking sig.-

nal andis non-conducting for. aspace and thev remainder of the receiving element .performsl in the `saine manner as was previously d'eseribedf for the condition when the cord, is `connected to a Hub jack..

YTransmission toward the :facility vrequires the operation vof .the teletypewriter sending keySen'd TTY which.permitsy the sending contactsjBBZ of theteletypewriterto operate sending relay ,lil

The signals transmitted pass `from the armature of sending relay E through resistance33, closed contacts 371 of key Send'TTY, back contacts 335 of relayBI,v contacts 3831er relay A2, close'd con` tac'tsof ringing key Ring, resistance .331, back. contacts ofrelay A!,.to the ring of thetelef typewriter cord, SEL. ring of. the legs jack ofthe connected facility and then over the sending 'leg SLwhich includes resistance i535 to operate the 4 S replay inthe facility in'lig. l. The sending relay in Fig. 1 is operated onan. inverse neutral" basis` as previouslyexplained.

.. It should be noted that the signals transmitted by sending relay addition to passing` through resistance-33t` on the wayto the ring of the cord',

also pass through the adjacent `back contacts 36! of .relay Al, closed contacts StZ of v-keyy FDXf-HDX, .and resistance 319 to the grid :off tube 3H. Thus outgoing signals `are transmitted..

into the receiving element and produce local copy.

Local lcopy is obtained provided transmission..

over the facility is on a half duplex basis. `The cordcircuit provides for full duplex as Well `as half ,duplex communication over the facility,H however, in ,order-that, the full duplextransmis.- sion capabilities or" the facilitymay be easilyv checked. Full duplex operation is obtainedby operating key FDX-HDX whichiopens the local copy circuit to prevent the outgoing signals .from

reaching thegrid of tube 3H. It also causes thef. release ofrelay'F which previous to the operation of the key has been heldoperatedY from ground onback v contacts 38'! ofv relay Al. -Release otre-l lay F .connects resistance Sie in .parallel with resistance 32|)` and lthis provides a lower impedance. between the tip `of the cord and theV grid fofftube.

3H. This `lower `impedance `to signals received` from the facility. helpsto'raise thepositivemark-ff` ranged for monitoring.

ig grid potential oftube 311'sc as tocomp'ensate for'the'reduction in the positive value of this pctential that is brought about by disconnecting the ring conductor of the cord which is at a positive potential, when idle, from the branch that includes resistance 319.

Teletypewriter cord circuit connected to Test :iaclc of concentration jack circuit 4The teletypewriter cord circuit per Fig. 4 is connected to the Test jack of a concentration jack circuit for monitoring signals from the associated facility or from the concentration group.

- It also provides for communicating with the concentration group when the facility is split ofi" from the group. Sleeve current flows from bat. tery 321 through the windings of relays A, B andl S, overV the sleeves of the cord circuit and the Test jack circuit andthroughthe winding of relay T in Fig. l to negative battery 139. Batteries 139 and 321 are poled opposing but since the voltage of battery 139 is about five times as high vas that of battery 321, current ows in a direction to operate the armature of polar relay A to the left to engage contact 384 but the armature of polar relay S remains in its right-hand position inengagement with contact 383. Owing to the high voltage of battery 139 in Fig. l the sleeve current is now suiiiciently high to ,operate marginal relay l-Zi` in Fig. 4. Relays A and B when thus'operated cause relays A1 and B1 to operate over obvious circuits and relay A1 connects ground through contact 358 to start theteletypewriter motor. Relay A2 is unoperated at this time `because its operating circuit is opened by the operation of relay Bl. With keys Split and Send TTY unoperated, the cord circuit is ar- When key FDX-HDX is in the normal condition, as shown, the cord per Fig. 4 will monitor signals from the facility and from the concentration group and when this key is operated it will monitor signals from the facility only. When key FDX-HDX is in the*y normal condition, as shown, signals are receivedfrom the facility over the tip of the cord and pass l through resistance 321i to the grid of `tube 311; Since transmission can be only hal-i' duplex, into Y a concentration group, when signals are being re- Y ceived from the facility the SL lead of vthe facility is at a steady positive potential which is the marking condition. The SL lead is connected to the ring of thecord circuit and extends through operated contacts 314 of relay B1, contacts 332 y of `full-duplex, half-duplex key, FDX-HDX and resistance 319 to the grid of tube 311. Battery 336 is now connected through operated contacts 310 of relay Bi and resistance 318 to the grid of tube 311 and this compensates for the presence of the steady positive potential from the SL 'lead on the grid. lTube 311 and the receiving element therefore respond in an unbiased manner to y transmission from the RL lead'of the facility. When the facility ceases to transmit signals, it sends a steady mark, positive potential, over 'the' tip of the cord and the concentration group may send toward the facility. Tube 311 and the receiving element will respond to signals from the concentration group which enter the cord circuit over the ring conductor.

When key FDX-HDX is operated to its open position, the connection between the ring of the cord circuit and the receiving lelement is opened at contact 362 so that the receiving element can no longer respond to signals fromlthe concentration -group. Opening of key FDX-HDX also Z2? ca'sesrlayF to release and 'connects vresi'stan'c'e 319 in parallel with resistance 320. This servesvv to reduce the impedance Vbetween the RL lead of Vthe facility and the grid of tube 311 and permits Key Split of the cord circuit is operated to .itsVVV closed position to split the facility off from the.v concentration group. This operates split relay'. SP from'ground through contacts 31|, 312 and 313 through the winding of relay SP to battery.. Relay SP when operated, short-circuits the wind-'k ingsof sleeve relays A, B and S by closing contact 314 and thereby increases the sleeve cur" rent sufficiently to cause the operation of relay` T in the sleeve of the Test jack circuit, Fig. 1..; RelaysAl :and Bl Which were initially operated.. from relays A and B when the cord was con-: nected to the Test jack will remain locked up from ground through contacts 311 of relay B1,

contacts 312 of operated key Split and through contacts, 315 and 316 in parallel to battery through the windings of relays AI and B1. Whenrelay T, Fig. 1, is operated the RL and SL legs are opened to disconnect the facility from the control circuit and the SL leg lead from the con'- GOS trol circuit is connected through contact 161 to,.- the ring of the Test jack. The RL leg lead of the'l control circuit remains connected to the tip of the Test jack. The cord circuit is not arranged for monitoring from the Aconcentration groupy with key Split operated. When the Split key is operated it is possible to communicate with the concentration group provided key Send TTY is rst operated and key FDX-HDX is released. With this arrangement signals from the concen-` tration group will pass through the control cir-...

cuit, over lead SL, through contacts 131 of relay T, Fig. l, to the ring of jack Test and over the ring of the cord circuit, through operated conf. tacts 314 of relay B1, closed contacts 3,32 of key.y FDX-HDX and through resistance 319to the gridoi tube 311. Tube Vlor the control circuit' of Fig. l is non-conducting for the marking conldition but positive battery 156 furnishes the nec-fy essary positive potential through resistancelq to send a marking pulse intoy the teletypewritercord circuit. For the spacing condition tube V4 conducts and reduces the positive potential furnished by battery 156 by a Sufficient. amount., to make tube 311, Fig 4, non-conducting.. As

communication is on a half duplex basis, a steady positive, marking, kpotential will be impressedY on the tip conductor oi the cord circuit by send-A: ing relay E while signals are being received from the concentration group This steady marking` v voltage will be applied to the grid of tube 31.1.. through resistance 320. As negative batteryr336' is connected to the grid of tube 311 through op.l erated contacts 310 of relay B1 and resistance 318v the marking and spacing grid voltageswill be approximately correct for unbiased reception', of signals from the concentration group as previ-.1. ously explained When signals cease tobe re' ceived `from the concentration group, a steady; marking,l positive, potential will be impressedf on the ring ofv the cord from positive battery 153 in the control circuit Signals transmitted toward the control circuit Will be sent from relay E, Fig 4, and these willpass through resistance. 3113, operated contacts 311 of key Send TTY, op: Y

erated contacts 31,8 of relay SP, which is operfh.,

ated'firomkey Split, through back `contacts 380 of=relay"A2 andthe operated contacts 359 of ren layI-iAI to th'eftipof the cord circuit From the tipioftthe cord circuit the outgoing signals will 4'pass over theiRL leg lead Fig. l to the control circuit. The voltages received by the control circuit will be positive for mark and negative for :space which will result in normal transmisfs'in-throughthe 'control circuit toward the concentration group. Local copy will bereceived by the receiving element of thecord circuit as the outgoing signals will be applied to the grid of tub`e"3|"| through resistance 32D. With steady positive potential applied to the grid through resistance 3|9 as previously mentioned and negative "battery 333connected tothefgrid via on- .gratedcontacts` 318 of relay Bl and resistance 318, `the .receiving element will respond .to local `signals on the tip of the cord circuit withreasonable. fidelity.

Itfwill be' noted that relay SQ is operated by Vthe operation of key Split over a circuit 'from groundion relay A: through contacts 332 and 383 and through the, winding of relay SQ to battery. Operation of relay SQ performs no useful'function at this time.

Teletypewritercord circuit connected` to teletypewriter jack Vof test signal cord circuit When the teletypewriter cord circuit per Fig. 4 1is`connected to the teletypewriter jack TTY of the `test signal cord circuit per Fig. 2, current iows'from ground onjthe sleeve of jack TTY through resistance 234, over the sleeve of the teletypewriter cord circuit, through the windings of Yrelays S, BandA to battery 327, Fig. 4. The current Value is too low to operate marginal relay'B and is in the wrong direction to operate polar relay A to engage contact 384. Polar relay S' wi1lbe operated to engage contact 386 and thecircuit arrangement within the teletype- Writer cord circuit will be similar to that which exists when the teletypewriter cord circuit is connected to the Legs jack of a concentration jack circuit, as previously explained.

The cord circuit may monitor test signals that are being transmitted through the test signal cord circuit. Thesesignals enter the teletypewriter cord circuit over the ring circuit, pass through back contacts 360 of relay AI which is released, through resistance 337 and back contacts 35| of relay AI, closed contacts 332 of key FDX-HDX and through resistance 3|9 to the grid of tube 3H. Negative battery 336 also is connected to this grid through back contacts 334 relay Al and resistance 3|8, and since a steady positive potential is connected to the grid from the' lRL leadofthe facility via the tip of the cord circuit and resistance 323, the tube will respond to the outgoing `test signals.

Full vduplex transmission tests may be conducted over the facility. This is accomplished by sending test signals over the test signal cord circuit in the usual manner while the distant. attendant sends test signals simultaneously using ,his` test signal cord circuit. Each attendant connects his teletypewriter' cord circuit to the telev typewriter jack of his test signal cord circuit and connects his transmission measuring set to the TMS'jack of his teletypewriter cord circuit. The key of each teletypewriter cord circuit' is operated so that the receiving element lire'spcn'd only-to the incoming signals which vfic'rn the Rllilea'd lof .the facility., over `the "ti-piofthe teletypewriter cord 'circuit and through lell owing tothe release of relay F, to the grid of tube=3`|'|.` Tube 3|'| responds to the signals and the receiving relay R repeats them to the transmissionmeasuring set which measures the bias and distortion of the incoming signals.

After transmission of test signals has been stopped, the local attendant may communicate with the Adistant attendant on a-half duplex basis. To do this it is `necessary to connect the teletypewriter cord circuit to the'teletypewriter jack of the test signal cord circuit and operate keys Send TTYv and Split. Operation of key Split causes relay SQ to operate from ground through contact 382 of relay A, contact 3533 of the `key Split and lthrough 4the winding ofrelay SQ to battery. `Signals from the distant attendant are received from the RL lead of the facility and passover the tip of thetest` signal cord circuit, and tip of the TTY cord circuit and through-resistance 328 to the grid of tube 3H. As this communication is half-duplex, sending relayE is transmitting `a mark while signals are incoming from the distant attendant. Thus at this time positive battery 335, Fig. 4, is connected through thelmarking contact and armature of relay E', through resistance 383, operated con'- tacts 3'|| of key Send TTY, back contacts 3675 of relay BI, back contacts 380 of relay A2, oper,- at'ed contacts of relay SQ and back contacts33 ofl relay A| tothe ring of the cord circuit. The positivepotential thus applied to the ring `of the cord circuit also is applied to the grid of tube 3|"| via resistance 331, back contacts 33| of relay Al, closed contacts 3320i key FDX-HDXand resistance 3| 9 to the grid and since 'negative battery v335is also'connected to the grid of tube3'l1 through back contacts 364 of relay Ai and resistance3|8 the tubeand receiving elementv will respond to signals from the distant attendant.

Signals are transmitted outward over thepath previously. described for the outgoing marking, positive, `signal and they pass from the ring of the teletypewriter cord circuit per Fig. 4 over the ring'of the test signal cord circuit of Fig. 2, and through resistance 20|, Fig. 2, to the sending A relay `of the facility. Owing to the presence of resistancekZl, Fig. 2, in the outgoing circuit it isnecessary that resistance 331.', Fig. 4, which is identical in value to resistance 28|, Fig. 2, be shortcircuited so as to avoid excessive reduction in current value. This is accomplished by the provision of relay SQ which as .previously notedis operatedby means of the Split key.

Local copy is provided. when communicating withthe distant attendant through thetest signal cord circuit.` lSteady positive, marking, potential is received from `the facility, through the tip of the test signal cord circuit and over the tip of the teletypewriter cord via resistance 326 to the grid of tube 3| l. The outgoing signals which pass over the ring of the cordalso `reach the grid ofthe tube through resistance 33T, back contacts 35| of relay Al, closed contacts 332 of le`y,FDX-HDX and .resistance 3|9. Since nega,- tive battery 333 is connected to the grid in the manner previously mentioned, the tube and receiving element will provide local copy.

Manual telegraph cord circuit The manual telegraph cord circuit is shown irLFig'. 5. Its purpose yis to connect a telegraph set consisting of a key and sounderto any ofthe various .circuits in the service Vboard forjmonitfie and communication. 1i is identica in design and operation to the attendants TTY vcord circuit except for the following diierences:

1. Relay S Fig. 4 is replaced by resistance 349 in Fig. 5.

2. The teletypewriter magnet V339 in Fig. 4 1s *replaced by the Morse sounder 439 in Fig. 5.

3. The teletypewriter sending contacts 352 in Fig. 4 are replaced by the Morse key 4t2, Fig. 5.

4. There is no teletypewriter motor and no teletypewriter motor starting arrangement is therefore required.

5. A second cord CIS 443 is provided in Fig.- 5.

AThis cord is used to answer and communicate with Morse stations that have called in for assistance.

Calling in signal cord per Fig. 5 and calling in signal circuitper Fig. 5A

Operation of cord CIS 443 will now be described:

.operates and applies ground through back contacts of relay SL to operate relay ML. Relay ML, when operated, applies battery to the ANS lamp circuit to light the lamp. It also connects ground through back contacts of relay SL to hold itself operated through released relay SL after the ground has been removed from the ,armature of relay SL by the release of relay CS.

The local attendant connects cord CIS, Fig. 5, a

to the CUST ANS jack of Fig. 5A and current ows from positive battery through the winding of relay SL and resistance 345 to the sleeve and over the sleeve lead inthe CIS cord circuit,

through varistor 342 and relay Si to ground. Varistor 342 is provided to prevent relay S! from operating if the CIS cord is connected to a Test or Hub jack inadvertently. Relays Sl, Fig. 5, and SL Fig. 5A both operate. Relay SL, Fig. 5A, in operating causes relay ML to Arelease and put out the ANS lamp. Relay SI arranges the circuit so that sending relay E, Fig. 5, can send signals through the line winding of relay R2 to the tip of the CIS cord. To do this, key

lSend TLG is operated and the signals pass from the sending contacts of relay E through resistance 303, left-hand operated contacts of key Send TLG, operated contacts 44'5 of relay SI, line winding of relay R2, over the tip conductor of cord CIS, over tip conductor of CUST ANS jack circuit, Fig. 5A, through the rings of jacks EQ and LP, lower winding of relay CS, resistance 341, through the Morse station set, potentiometer 348, upper winding of relay CS, tip conductors of jacks LP and EQ to negative battery in the loop repeater. Transmission is on a neutral ibasis and the outgoing signalsare transmitted to the hub through the loop repeater and control circuit in addition to being received at the Morse station. Local copy is obtained from polar relay R2 which follows the outgoing signals. The lower winding of this relay serves as a biasing winding and current ow from battery 345 through Ythe winding to ground tends to operate the relay to spacing. Relay R2 sends neutral signalsirom battery 344 thro-ugh the marking contact and armature of relay R2 and operated contacts 449 of relay Si to Morse sounder 439 for local copy.

s When the customers station in Fig. 5A sends to the service board attendant, the loop is opened and closed which interrupts the loop current which hows from positive battery 395, Fig. 5,

through the sending circuit of Fig. 5 and over the loop to negative battery in the loop repeater. These signals actuate the receiving relaynot shown, in the loop repeater which repeats them via the control circuit into the concentration group. Relay R2of Fig. 5 also responds to the signals from the station and repeats them on a neutral basis through operated contacts 449 oi relay SI and the winding of telegraph sounder 43ste ground thereby actuating the sounder.

Loop monitoring circuit The loop monitoring circuit is shown in'Fig. 6.

It is used for monitoring signals in subscriber loops, and also provides means for sending and receiving test signals through a loop repeater from the loop side as a means of testing the re'- peater and control circuit between the hub and the loop.

The method of monitoring in a loop will now be described. The loop monitoring circuit ex" tends from the service board to the line position of the telegraph test board. Jacks Send'FOX, MON Polar, MON Neutral and Disc appear in the Yface of the service board and Jacks RGL and RGEQ, key RLS and lamp DISC appear in the line position. When the attendant at the service board wishes to monitor, he requests the line posi; tion attendant to patch jacks RGL and RGEQ, to the LP and EQ jacks of the loop on which he wishes to monitor. The LP and EQ jack circuit of a loop repeater is shown in Fig. 6A. Y The line board attendant uses two loop patching cord cir.- cuits similar to that shown in Fig. 6B, to make the patch. He patches the LP cord of therst cord circuit to the RGL jack of the loop monitoring circuit and the EQ cord to the EQ jack ofthe loop and equipment jack circuit. Next he patches the LP cord ofthe second oord circuit to the VLP jack of the loop and equipment jack circuit and the EQ cord to the RGEQ jack of the loop monitoring circuit. Fig. 5A shows a loop repeater with its LP and EQ jack circuit, calling-in relay and loop. The service board attendant connects his teletypewritercord circuit to the MON Polar jack if loop transmission is polar from the loop repeater to station, i. e. if the loop returns to ground instead of to the positive battery shown in the loop repeater of Fig. 5A. For neutral transmis'- sion such as that shown in Fig. 5A where the loop returns to positive battery in the loop repeatenJ the attendant connects his lteletypewriter cord circuit to the MON Neutral jack. In either case the loop circuit will be closed through the line winding of relay R and the back contactsl of relay FS, Fig. 6. If transmission is polar it is only polar when transmission is from theloop repeater to the station and it is always neutral when trans'- mission is from the station to the loop repeater. Connection of the TTY cord circuit to the MON Polar jack causes the Vbias circuit of relay Rte be left open and the teletypewriter cord cannot monitor' signals from the station to the loop repeater unless it is disconnected from the MON Polar and connected to the MON Neutral jack-l. Connection of the TTY cord circuit to the MON Neutral jack causes sleeve relay S to operate. and close the bias circuit of the R relay. Relay R will now respond to neutral signals in the: loop.

- In any case relay R will repeat the signals through 29 teletypewriterfcord.circuit ris" connected ,-to" any ja'ckxof ,theloop monitoring circuit jacksgatthe service board, .negativebattery` of highA potential on' the sleeve of thejack causes sleeve relays 'A andiB of'. the cord` circuit `per Fig.i4to operate. The 'cord `circuit is therefore arrangedv in the same-manner as Whentit is connected to the `Test jack of a .concentration jack circuit. Signalsmanner;

When it is desirable to send test signals through a'- loop repeater and control circuit and make transmission measurements on the hub, the RGL jack of the loop monitoring circuit Fig; 6 is patched to the EQ jack per Fig, 5A of the loop repeater under test using a loop patching cord i circuit per Fig. 6B. The SIGS jack of a test signal supply circuit per Fig. 3 is patched to the Send FOX jack of the loop Lmonitoring. circuit by meansoftaleg patching cordcircuit per Fig. 1B. Sleeve current flows from battery 493 through.` the winding of relay! FS over the sleeve circuit of ,1`aclr Eer1.d FOX, Fig..6, the leg patching cord `circuit.` andthe test-signal Asupply circuit, Fig43, 'through thewinding `of. relays A, BV andS to battery 2Min Fig..3. Relay FS of the loop monitoring circuit operates andcloses the circuit between the tips of jacks RGL and Send FOX Which includes resistance 4M. Relays A and B of the test signal supply circuit per Fig. 3 operate and cause relays A! and AB to operate. Relays Al and AB start the multiple sender or other source of test signals and the signals pass through resistance 222, Fig. 3, operated contacts of relays AI and AB, tip of jack SIG-S, tip oi the leg patching cord circuit, tip of jack Send FOX 1'.

of the loop monitoring circuit per Fig. 6, resistance 4M, and operated contacts of relay FS to the tip of the jack RGL. From the tip of jack RGL of Fig. 6 the signals pass over the tip of the loop patching cord circuit, Fig. 6B, to the tip of jack EQ, Fig. 6A, and into the loop repeater which repeats the signals via the control circuit to the receiving hub.

Test signals may be sent through the control circuit and loop repeater from the sending hub to theloop side and transmission measurements may be made on the loop side by means of the loop monitoring circuit. In this case the loop monitoring circuit is patched to the LP and EQ jacks of the loop repeater under test in the same manner as previously described for the monitoring condition. This patch requires two loop patching cord circuits per Fig. 6B. The patch between the test signal supply circuit and the Send FOX jack is removed to permit the FS relay to be released and thereby close the loop for monitoring.

What is claimed is:

1. In` combination in a telegraph system, a telegraph concentration group circuit comprising a plurality of telegraph lines each interconnected through an individual telegraph line repeater, an individual jack circuit and a telegraph hub line repeater electronic control circuit, for regulating the direction of transmission, said interconnection-by means of -an 'individual.receiving legand anindividualsending legto aqsinglefreceiving hub anda singlesending hub common'to allof said'telegraphlines, andmeans comprising an a'ttendants cord circuit having a telegraph transmitter andxfreceiver therein forccommuni:u eating either over an individual one of said plurality of lines or through the hub ofY saidcon# centration to all of said lines.

2. In combination .in a telegraph system; 'a telegraph concentration `group circuit 'comprising a plurality of telegraph lines each interconnected through anljindividual telegraphline repeater,. an individualjack circuit and. a` telegraph hubline repeater electronic control circuit,'for regulatingY the direction of transmission, said in'-` terconnection` by means or an individual receiving leg and an individual sending leg toa single receiving hub and a'single sending hub commento all of said telegraph lines, a spare telegraph line equipped with an individual telegraph line'repeater and anv individual jack circuit, aspare telegraph hub lineI repeater electronic control circuit, and m'eansincluding a patching cord circuit for interconnectingsaid spare line through said spare control circuit tosaid receiving :hub and said sendinghub.

3. In combination ina telegraphsystem, a. telegraph concentration group circuit comprising a plurality of telegraph lines eachlinterconnected through an individual` telegraph line repeater', an individual jack circuit and a telegraph hub linerepeater electronic control circuit,.for`regu`- lating the direction of transmission, said inter.- connection-*by means ofan individual receiving leg and an individual sending leg, to a singlexree ceiving hub anda single sending hub common `to all of said telegraph lines, a spare telegraph line equipped with an individual telegraph line repeater and an individual jack circuit, means comprising a patching cord and said jack circuits for interconnecting said spare line through any selected one of said electronic control circuits to said receiving hub and said sending hub and releasing means responsive to the interconnection of said spare line for disconnecting the line regularly interconnected through said selected control circuit to said hubs.

4. In combination in a telegraph system, a telegraph concentration group circuit comprising a plurality of telegraph lines and loops each interconnected through an individual telegraph line repeater, an individual jack circuit and a telegraph hub line repeater electronic control circuit, for regulating the direction of transmission, said interconnection by means of an individual receiving leg and an individual sending leg to a single receiving hub and a single sending hub common to all of said telegraph lines and loops, a hub jack connected to said hubs, :a source of test signals and a test signal cord circuit for interconnecting' said test signal source through any of said individual jack circuits to its associated line or loop and another cord circuit for connecting said source to said concentration group circuit through said hub jack.

5. In combination in a telegraph system, a telegraph concentration group circuit comprising a plurality of telegraph lines each interconnected through an individual telegraph line repeater, an individual jack circuit and a telegraph hub line repeater electronic control circuit, for regulating the direction of transmission, said interconnection by means of an individual receiving leg and an individual sending leg to a 

